In our current media landscape, it apparently takes a magazine like Rolling Stone — in an issue with Justin Bieber on the cover — to offer a writer like Bill McKibben the opportunity to spell out the facts, in cold hard arithmetic, for a mass audience. McKibben's landmark article this past summer, "Global Warming's Terrifying New Math," boiled the hard truth about climate down to three stark numbers:

• Two degrees Celsius: the amount, according to international consensus, that we can raise the global average temperature above preindustrial levels and still maintain a so-called "safe" climate, beyond which all bets are off. "Safe," of course, depends on where you live. We've already raised it almost one degree, with disastrous results; if you live in Africa, or Kiribati, one degree is too much.

• 565 gigatons: the amount of CO2 scientists agree we can still pump into the atmosphere and hope to remain below the two-degree threshold.

• 2795 gigatons: the amount of CO2 contained in the world's proven fossil-fuel reserves, which the fossil-fuel industry shows every intention of extracting and burning.

The bottom line: we have to find a way to leave 80 percent of accessible fossil fuels in the ground, forever, and make a rapid shift to clean energy, if we're going to avoid the very real risk of catastrophic climate change within this century. When you get a grip on those numbers, something like the Keystone protest — driven by the idea that the Alberta tar sands, the planet's second-largest pool of carbon, should be off-limits — comes into focus. It's more than math: it's a moral imperative. That's why 1253 people were willing to get arrested in front of the White House in order to stop that pipeline, even temporarily.

"Unsafe" climate change is not a distant threat. It's here, now. We've fundamentally altered the planet's life-support system, and conditions are going to get much worse. If you've enjoyed this year's record heat, wildfires, drought, and spiking global food prices — if you enjoy monster storms like Sandy — get used to it.

Of course there's uncertainty about exactly how these changes will unfold. There will always be uncertainty in anything as complex as climate science. But as MIT's Emanuel has said, "Uncertainty doesn't translate into 'no worries, mate.' " In fact, it's the opposite. Uncertainty, he notes, "is a double-edged sword." It's possible, Emanuel and his colleagues acknowledge, that the impacts of climate change will be less severe, and arrive more slowly, than the most sophisticated models predict. But it's equallyprobable that the impacts will be much more severe, and arrive much faster, than predicted. So far, mounting evidence like the rapid melting of the Arctic ice cap — one of the planet's largest physical features, which reached its lowest extent ever recorded this summer, blowing away all predictions — suggest that the latter may well be the case.

What's more, as Emanuel and others go on to point out, because of the inherent inertia of the planet's climate system, and the sheer amount of CO2 already in the atmosphere, our "window of opportunity" to prevent catastrophic warming is extremely narrow. It may even have already closed. We don't know. According to the IPCC, global emissions need to drop at least 25 percent below 1990 levels by 2020 — eight years from now — and at least 80 percent by 2050, if we're going to have a shot at maintaining a livable climate.